Allen&#39;s hummingbird (Selasphorus sasin) is a species of hummingbird. It is a small bird, with mature adults reaching only 3 to 3.5 in (76 to 89 mm) in length. The male has a green back and forehead, with rust-colored rufous flanks, rump, and tail. The male&#39;s throat is also an iridescent orange-red. The female and immature Allen&#39;s hummingbirds are similarly colored, but lack the iridescent throat patch, instead having a series of speckles on their throats. Females are mostly green, featuring rufous color only on the tail, which also has white tips. Immature Allen&#39;s hummingbirds are so similar to the female rufous hummingbird, the two are almost indistinguishable in the field. Both species&#39; breeding seasons and ranges are common factors used to differentiate between the two species in a particular geographical area.